1. Field of the Invention
The present invention relates to a backlight unit that can perform local dimming, has a structure that prevents light leakage and has a structure for cooling, and an image display apparatus employing the backlight unit.
2. Description of the Related Art
Recently, image display apparatuses, using a backlight unit, have been employing a local dimming technique by turning off dark image areas in the backlight unit to improve the contrast ratio in a region between a bright area and a dark area. The backlight unit capable of performing the local dimming can be classified as a carbon nanotube (CNT)-backlight unit using a CNT as an emitter or as a light emitting diode (LED)-backlight unit using a LED as a point light source.
A CNT-backlight unit includes a rear substrate, CNT emitters, a front substrate that is transparent, and a fluorescent layer that is arranged on a lower surface of the front substrate. Black matrixes, that are used to divide the fluorescent layer into unit blocks that are for local dimming, are arranged in the fluorescent layer. An anode is arranged between the fluorescent layer, that is a light emitting unit, and the front substrate. Also, the CNT-backlight unit can further include a diffusion plate to uniformly emitting light. Light uniformly emitted through the diffusion plate is provided to a display panel, such as a liquid crystal display (LCD) panel.
An LED-backlight unit includes a base, a plurality of point light sources arranged on the base, and a diffusion plate facing the point light sources. In the LED-backlight unit, the point light sources are mainly LEDs and are arranged in a two-dimensional array on the base. Some of the point light sources are gathered to form unit blocks, that are for local dimming. In this case, the point light sources that are included in one unit block flicker together. The diffusion plate uniformly emits light radiated from the point light sources to a display panel, such as a Liquid Crystal Display (LCD) panel.
However, in the CNT-backlight unit and the LED-backlight unit, light leakage occurs when light reaches the diffusion plates during local dimming, thereby influencing adjacent blocks. As a result, the contrast ratio is decreased in the boundary between the blocks. In the case of the CNT-backlight unit, a temperature of an anode (not shown) of a light emitting unit is increased to more than 80° C. Accordingly, if the anode is not cooled, the CNT-backlight unit can be damaged due to thermal shock.